Add support for enums.

tests/enum-5.ispc

@@ -0,0 +1,26 @@
+
+enum Foo {
+    ZERO,
+    ONE,
+    TWO,
+    TEN = 10,
+    ELEVEN
+};
+
+enum Bar {
+    FIFTY = 50,
+    THOUSAND = 1000
+};
+
+export uniform int width() { return programCount; }
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex];
+    uniform Foo f = ONE;
+    int g = ++f;
+    RET[programIndex] = g-1;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1;
+}